Re: [PATCH v7 06/25] rust: add `compiler_builtins` crate

From: Gary Guo <gary@garyguo.net>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Miguel Ojeda <ojeda@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
	Jarkko Sakkinen <jarkko@kernel.org>,
	Alex Gaynor <alex.gaynor@gmail.com>,
	Wedson Almeida Filho <wedsonaf@google.com>,
	Sven Van Asbroeck <thesven73@gmail.com>
Subject: Re: [PATCH v7 06/25] rust: add `compiler_builtins` crate
Date: Tue, 24 May 2022 00:41:56 +0100	[thread overview]
Message-ID: <20220524004156.0000790e@garyguo.net> (raw)
In-Reply-To: <CAKwvOd=mFhxjKRP_qt3Yu69dj_P6VUMSUSQm7fY6yS2bsO8Y2w@mail.gmail.com>

On Mon, 23 May 2022 11:37:16 -0700
Nick Desaulniers <ndesaulniers@google.com> wrote:

> Also, I'm not sure my concern about explicit build failures for C code
> was ever addressed?  We have a constant problem with `long long`
> division on ARCH=arm32 and ARCH=i386 in C code.
> https://lore.kernel.org/lkml/CAKwvOdk+A2PBdjSFVUhj4xyCGCKujtej1uPgywQgrKPiK2ksPw@mail.gmail.com/
>
> > +#[cfg(target_arch = "arm")]
> > +define_panicking_intrinsics!("`u64` division/modulo should not be
> > used", {
> > +    __aeabi_uldivmod,
> > +    __mulodi4,
> > +});

Starting in LLVM 14 (used in Rust 1.60+), __mulodi4 will no longer be
generated. So that can be removed.

As for __aeabi_uldivmod, is there any reason that it can't just be
defined in arch/arm/lib? There are quite a few __aeabi functions already
defined there.

The source of __aeabi_uldivmod in compiler-rt seems quite simple, just
delegating to __uldivmoddi4. I think just changing that to
div64_u64_rem should do the job?

https://android.googlesource.com/toolchain/compiler-rt/+/release_32/lib/arm/aeabi_uldivmod.S

- Gary